1. Field of the Invention
This invention relates generally to the field of systems capable of internally generating control signals and a method and apparatus for inhibiting the effect of generation of those signals unless appropriately authorized. More particularly, this invention relates to a microprocessor controlled radio pager wherein the microprocessor has the capability of turning itself and the pager off, and a circuit arrangement and method for protecting the radio pager against inadvertently being turned off as a result of loss of program control.
2. Background of the Invention
It is frequently desirable to utilize a microprocessor or microcomputer in a paging receiver to control various decoding, alerting and power control functions. Since a paging receiver should be small, compact, and light weight, and therefore easily and conveniently carried on the person, the microcomputer provides a compact and powerful way to implement and actuate complex functions while maintaining small size.
It is frequently desirable to utilize so called "popple" or "snap dome" type switches such as those manufactured by K. B. Denver, Inc. of 451 Oak Street, Frederick, Colo., due to their small size, light weight, low cost and high reliability. These switches are normally open momentary type switches which may be readily interfaced with a microcomputer in order to control the various functions of the paging receiver. One example of such functions is to utilize a single momentary switch for a "push-on/push-off" function.
In such a computer controlled paging receiver, particularly where snap dome or other momentary switches are utilized, it is advantageous to have the microcomputer perform various functions, including the function of turning the receiver on and off upon receipt of an appropriate control input from an ON/OFF switch. Curiously enough, this combination of elements carries with it a possibility that the microcomputer could turn itself off.
In a simple implementation of such a system, the microcomputer periodically inspects the status of an OFF switch to determine if it has been actuated. If it has been actuated, the microcomputer can then send a control signal to the power source of the pager, which might be a switchable D.C. to D.C. converter. The power source then disables power to all circuits in the paging receiver requiring full power. It is likely that such a system would also require standby power to power certain standby circuits in the pager which allow the pager to come back to life upon recognition of the actuation of an ON switch.
In the system as described, the microcomputer has the ability to turn off the system as the microcomputer sees fit. It is possible due to the presence of high energy radio frequency fields, alpha particles, static discharge, loss of computer synchronization, etc. that the computer may deviate from the programmed operation and could erroneously generate a turn off signal without having received instructions to do so as a result of an OFF switch actuation. For systems which require a high level of system integrity over long periods of time, whether it is a paging receiver or other microcomputer controlled system, this may be an unacceptable circumstance. In the case of a paging receiver, the pager could cut itself off, thereby losing messages stored in memory and/or missing future paging messages until the user discovers that the pager is off. This seriously detracts from the utility, convenience and reliability of communications for which the pager is intended. In other computer controlled systems, the results of a self-initiated unchecked control signal generation can even be far more disastrous.